1. Field of the Invention
The present invention relates to a dopant material using a carbon nanotube, a method for manufacturing the dopant material, and a semiconductor device such as a transistor which is preferable for p-type and n-type conduction semiconductor channels.
2. Description of the Related Art
A carbon nanotube has a tubular graphite structure with a diameter of some nanometers and a length of some hundreds nanometers to some micrometers. Depending on chirality and the diameter, there are metal-type carbon nanotubes and semiconductor-type carbon nanotubes. The semiconductor-type carbon nanotube can be used as a transistor channel. The semiconductor-type carbon nanotube is particularly important as a semiconductor material to be used as post silicon material in terms of device application, since it has more than ten times drift mobility than that of silicon and the structure of the band gap can be controlled by the diameter and chirality, etc. Ever since there was a report on a field-effect transistor comprising a carbon nanotube as a channel (non-patent literature 1), there have been studies and developments actively conducted on carbon nanotube transistors all over the world.
For practical implementation of the carbon nanotube transistor, it is necessary to develop techniques on several elements. Examples may be a technique for controlling diameter, position and orientation of carbon nanotube, a technique for selecting carbon nanotube metal/semiconductor, a low-resistance ohmic electrode technique, a technique for fabricating high-performance gate insulator film, a doping technique for carbon nanotube, etc. Particularly, the doping technique is important for controlling the conduction types such as p-type conduction and n-type conduction of the carbon nanotube transistor, carrier density, threshold value of the gate voltage, and the like, and it is the key stone to achieve high-performance and high-speed devices.
In general, “doping” means to add a foreign substance for controlling the property of a semiconductor (particularly, for controlling the conduction type of a semiconductor). There are two types in the conduction types of the semiconductor, one of which is n-type conduction and the other is p-type conduction. The semiconductor exhibiting n-type conduction is referred to as an n-type semiconductor, in which electrons supplied to the conduction band of the semiconductor from a donor (electron donor, n-type dopant) as a foreign substance in the n-type semiconductor perform electrical conduction. The semiconductor exhibiting p-type conduction is referred to as a p-type semiconductor, in which holes that are generated due to deprivation of electrons from a valence band of the p-type semiconductor by an acceptor (electron acceptor, p-type dopant) as a foreign substance perform electrical conduction. Referring to the carbon nanotube, the n-type conduction carbon nanotube or the p-type conduction carbon nanotube can be produced by doping an appropriate donor or acceptor.
As a conventional technique for producing the n-type conduction carbon nanotube, for example, there have been reported a method of potassium (K) vapor evaporation (see non-patent literature 2) and a method of vacuum heat treatment (see non-patent literature 3). However, carbon nanotube channels fabricated by those methods are chemically unstable in the air, thereby determined unsuitable for use in a device of stable action. As another conventional methods for producing the n-type conduction carbon nanotube, there are known a method of supplying polymer materials containing imine group from outside the carbon nanotube (see non-patent literature 4) and a method of introducing organic molecules as the donor with relatively low ionization energy into hollow of the carbon nanotube (see non-patent literature 1). However, it is very difficult with these methods to control the doping concentration, i.e. the carrier density within the carbon nanotube channel. Specifically, it is necessary with the latter method to prepare a carbon nanotube channel in which donor is filled in advance into the hollow of the carbon nanotube when fabricating a transistor. Thus, a transistor fabricating method which is essential for integration of the carbon nanotube transistor, e.g. doping on the transistor by growing the carbon nanotube in situ as disclosed in patent literature 2, cannot be applied, and specific operation property of the transistor and the like fabricated by the doping method is not presented.
Further, as a conventional technique for producing the p-type conduction carbon nanotube, there has been reported a method of naturally attaching oxygen/water molecules which are considered as the source for hole supply from the atmosphere without applying a special treatment to the carbon nanotube. However, in a device that uses the carbon nanotube fabricated by this method, the property of the carbon nanotube transistor changes depending on the external environment. Therefore, it is not reliable. As still another conventional technique for producing the p-type conduction carbon nanotube, there is known a method of introducing organic molecules with relatively large electron affinity into the hollow of the carbon nanotube (see patent literature 1). However, it has the same disadvantages as those of the above-described method that it is very difficult to control the carrier density, inapplicable to the carbon nanotube transistor grown in situ, and that the specific device action is not clarified.
[Non-Patent Literature 1] Nature, vol. 393, pp 49-52, 1998
[Non-Patent Literature 2] Physical Review B, vol. 61, pp R10606-R10608, 2000
[Non-Patent Literature 3] Physical Review, Letters, vol. 87, pp 256805-256808, 2001
[Non-Patent Literature 4] Journal of American Chemical Society, vol. 123, pp 11512-11513, 2001
[Patent Literature 1] Japanese Patent Unexamined Publication 2004-311733 (FIG. 1, FIG. 3)
[Patent Literature 2] Japanese Patent Unexamined Publication 2004-67413 (FIG. 17)